A great many methods have previously been proposed for recovering oil from oil shale, nearly all of which involve some form of pyrolytic eduction. However, for a variety of economic reasons, none of these methods has yet proven competitive with the production of mineral oils from petroleum sources. It may be said in general that the principal overall difficulty resides in recovering essentially all of the hydrocarbonaceous material from the shale without resorting to prohibitively expensive methods. Since shale rock usually contains only about 20-80 gallons of oil per ton, it is a practical necessity to recover at least about 80-90 percent of this oil, and in the most economical possible fashion. It is not essential however that all of this oil be recovered as liquid product; combustible gaseous products can also be economically utilized in the eduction process itself, or in other ways. The overall objective remains to recover the maximum possible energy values at minimum expense. This includes a maximum conservation of heat in the process.
Due to the expense involved in transporting raw shale rock and disposing of spent shale, it is a practical necessity in most eduction processes that the reporting facilities be located closely adjacent to the mining site. These mining sites are nearly always located some distance from refining facilities. Transporting the educted shale oil to such refining facilities has proven to be a problem, mainly because the high nitrogen content of shale oils renders them incompatible with crude petroleum and unsuited for transportation in common carrier pipelines. It should be highly desirable to provide at the retorting site at least sufficient refining facilities to convert the crude shale oil to a pipelineable material. The most desirable refining process for this purpose would be a catalytic hydrofining unit, but this has generally been considered impractical because of the expense involved in providing a suitable source of hydrogen. According to my process, a suitable such hydrogen source is integrated into the retorting system itself, thus providing the needed hydrogen at minimum expense.
Briefly, in my process, spent, hot, coke-containing shale produced for example by the method described in my prior U.S. Pat. No. 3,361,644, is gravitated downwardly through a gasifier in which is maintained a combustion-gasification zone, and a mixture of oxygen-containing gas and steam is passed upwardly countercurrently therethrough. The amount of oxygen injected is controlled to burn only sufficient coke to produce the amount of heat required to gasify a predetermined remaining portion of coke via the endothermic gasification reactions such as: ##STR1##
The resulting essentially oxygen-free product gas will be designated herein as "water gas" even though that term conventionally refers to mixtures much richer in carbon monoxide and leaner in carbon dioxide than those produced herein.
It should be noted at this point that in order to maintain optimum temperature profiles in the gasifier, it is necessary to provide therein a total gaseous heat carrying capacity considerably greater than that which would be provided simply by the stoichiometric quantities of steam and oxidizing gas required to gasify the coke on the spent shale. The total heat capacity of the gas stream in the gasifier should be approximately equal to the heat capacity of the spent shale supplied thereto. For example, if the spent shale rate is 200 tons per hour and the shale gives up 112,000 Btu's per hour per degree Fahrenheit in cooling, the quantity of gas moving up the gasifier should be sufficient to absorb 112,000 Btu's per hour per degree Fahrenheit in the process of heating up.
To provide the necessary heat capacity in the gas stream, it is desirable to utilize an excess of water vapor, which not only provides high heat-carrying capacity but also is advantageous in shifting the equilibrium of reaction (5) above to the right. However, one problem involved in the use of excess steam is that of conserving the heat of vaporization of the unconsumed excess thereof in the water gas product. To produce a useful product gas, most of the excess steam must ultimately be condensed out, involving a substantial evolution of 212.degree. F-minus heat, which cannot be economically utilized in the retorting or gasification operations. I have found that a substantial proportion of this heat loss can be avoided by withdrawing a recycle stream of the water gas product (which is ordinarily at a temperature of about 850.degree.- 1100.degree. F), passing it through a steam generator to effect simultaneous cooling and steam-enrichment thereof, and then returning the enriched gas to the lower portion of the gasifier. By operating in this manner, a substantial proportion of the steam required for heat carrying in the gasifier is never condensed, with resultant heat savings amounting to about 400,000 to 500,000 Btu's per ton of raw shale.
It is noteworthy that although the recycle water gas stream contains hydrogen and must pass through the combustion-gasification zone in which hydrogen-burning temperatures prevail, there is substantially no net reduction in overall hydrogen yields. With the oxygen supply limited so that only a portion of the coke can be burned, it appears that the kinetics of the various reactions occurring in the combustion-gasification zone are such that the heat produced from any burning of hydrogen merely relieves an equivalent proportion of carbon from its heat-producing duty and renders it available for hydrogen production via the endothermic gasification reactions. Consequently there is substantially no net reduction in overall hydrogen production.
The net unrecycled product portion of water gas from the gasification zone is mixed with a major proportion of preheated retort off-gas comprising mainly carbon dioxide, hydrogen and light hydrocarbons (and nitrogen if air is used as the oxidizing gas), and the resulting hot gas mixture is then passed through the retorting zone countercurrently to a stream of crushed shale rock to effect oil eduction by pyrolysis of kerogens. The large volume of retort recycle gas is required in order that the necessary heat for eduction can be supplied without preheating the smaller volume of water gas from the gasification zone to such high temperatures as to bring about undue cracking of the educted oil. High temperature gases are also undesirable because heat is unnecessarily wasted in the endothermic decomposition of mineral carbonates in the spent shale.
The combined eduction gases and product oil are then recovered from the eduction zone and separated as described more in detail hereinafter. When air is used as the oxidizing gas in the gasification zone, the retort off-gas will comprise a large proportion of nitrogen and will have a relatively low heating value. Where substantially pure oxygen is employed in the gasification zone, a high Btu off-gas is obtained which is much richer in hydrogen, and from which carbon dioxide can be separated to produce a 70-90 percent hydrogen gas stream suitable for use in a catalytic hydrofiner to upgrade the liquid shale oil produced.
A particularly desirable modification of my process involves the use of at least one retort operating with an oxygen gasification zone, in combination with at least one retort operating with an air gasification zone. The former retort or retorts produce hydrogen for hydrofining, while the latter produces low Btu heating gas to supply the thermal requirements of the combined retorting and gasification systems.
I am aware that combined shale retorting-gasification processes have previously been proposed, for example those described in U.S. Pat. Nos. 3,577,338 and 3,736,247. However, in these processes the raw shale is passed downwardly through the retorting zone and continuously into the gasification zone, while the necessary recycle gas for eduction (comprised of product gas from the retorting zone) is introduced at the bottom of the gasification zone and passed upwardly, first through the gasification zone and then through the eduction zone. Steam and oxygen are introduced at intermediate levels to effect gasification. I have found these procedures to be disadvantageous from several standpoints. Firstly, it is difficult to maintain a sharp separation between the eduction and the gasification zones. Educted oil in the top of the retort tends to reflux downwardly into the hotter gasification zone where cracking may occur, and where they may be unconsumed oxygen. But even more importantly, I have found that it is disadvantageous to pass retort off-gas through the gasification zone, for two principal reasons:
Firstly, prior to reheating for recycle, this product gas has of necessity been cooled to condense out most of the vaporized retort hydrocarbons, and concomitantly water vapor is condensed with the result that only a low equilibrium concentration thereof remains in the recycle gas. As noted above, in the gasification zone it is desirable to maintain a high partial pressure of water vapor. If sufficient steam is added to the retort recycle gas to achieve this objective in the above noted prior art processes, there would be excess gaseous heat carrying capacity, both in the gasification and retorting zones. Moreover, inordinate amounts of steam would need to be generated and condensed with each pass through the system, necessarily entailing large heat losses.
Secondary, the retort product gas contains about 2-10 volume percent of light hydrocarbons which are burned preferentially in the combustion-gasification zone. The only way, in this operation, that the coke on spent shale can produce a useful product is by reaction with steam and carbon dioxide. It has been found that the hydrocarbons in the recycle gas suppress the gasification reactions so that less than half of the coke on spent shale can be gasified in this manner. As a result, more than 10% of the recoverable energy in the raw shale is lost to the ash discard. These considerations appear not to have been properly evaluated in prior art gasification-retorting processes.
It is an important feature of my invention to introduce the retort recycle gas into an essentially oxygen-free transition zone maintained between the gasification and eduction zones, and in which the spent shale has not yet been heated to above about 1100.degree. F. This permits control of the total volume of gas flowing into the pyrolysis zone so that the optimum gas to solids ratio can be obtained therein. The temperature of the gas entering the pyrolysis zone also can be controlled and optimized. The temperature of the total gas (water gas plus retort recycle gas) entering the pyrolysis zone can be adjusted by controlling the temperature of the retort recycle gas.
Finally, I am unaware of any prior art moving-bed gasification process which embodies the highly advantageous heat-conserving feature of my invention, involving the recycle of a portion of the gasifier product gas through a steam generator and thence back into the lower section of the gasifier.